The present invention relates to the use of a cryostat for maintaining a specimen at a low temperature so as to allow scientific examination or experiment to be accurately performed. More particularly, the present invention is concerned with a nozzle for use in such a cryostat.
X-ray crystallography is well-known as one particular scientific method in which a specimen may be subjected to a beam of x-rays with the x-ray diffraction patterns obtained being indicative of the crystalline structure of the specimen. Another known method is neutron crystallography. In general, biological samples subjected to such methods are more robust if they are frozen, and uniform results are more likely to be obtained if their temperature is kept stable. Thus, chilling of samples, for example, by the use of a cryostat, and particularly by open stream cooling, is well known. Very cold temperatures, for example those at less than the 77.4 degrees Kelvin boiling point of liquid nitrogen, are highly desirable, since they allow crystal phase changes to be studied. Such very cold temperatures are obtainable by the use of a xe2x80x98cryogasxe2x80x99, a term which in this specification is defined as meaning helium, neon or hydrogen in a gaseous or liquid phase.
U.S. Pat. No. 5,653,113 discloses a cooling system comprising a nozzle pipe for jetting low-temperature gaseous nitrogen from a tip end opening thereof, the nozzle pipe being disposed inside a cooling chamber supplied with ordinary temperature nitrogen gas. However, the system has the disadvantage of not being adapted to cool samples below the boiling point of nitrogen to temperatures at which x-ray crystallography is particularly fast and accurate.
U.S. Pat. No. 4,295,339 discloses a cryostatic system utilising a liquefied gas in which an object to be cooled in a thermostatic chamber is cooled initially by a liquid coolant at a cryogenic temperature and then by a gaseous coolant. The coolant is typically nitrogen or carbon dioxide, thus the system suffers the same disadvantages as that disclosed by U.S. Pat. No. 5,653,113, when the coolant is in its gaseous form.
It is an object of the present invention to provide an improved method of using a cryogas cryostat and an improved nozzle for use in such a cryogas cryostat.
In accordance with the present invention, a method of using a cryogas cryostat comprises supplying a stream of cryogas over a specimen, and surrounding said stream in the vicinity of the specimen by a dry annular flow of cryogas at ambient temperature, characterised in that the stream of cryogas is helium at a temperature of between 4.2 and 77.4 degrees Kelvin and the dry annular flow of cryogas is the same cryogas.
Preferably in the vicinity of the specimen there is no solid barrier between said stream and said annular flow, and the speed of flow of said stream is substantially the same as the speed of said annular flow.
Preferably further in the vicinity of the specimen there is no barrier at all between said stream and said annular flow, whereby said stream is supplied adjacent the surrounding annular flow.
Preferably further in the vicinity of the specimen there is supplied an outer annular dry ambient airflow surrounding the annular flow of ambient temperature cryogas.
Preferably further the speed of flow of said stream in the vicinity of the specimen is at least 25 cms/second.
Preferably further the speed of flow of said stream in the vicinity of the specimen is less than 10 meters/second.
Preferably further the speed of flow of said stream in the vicinity of the specimen is between about 50 cms and 1 meter/second.
Preferably the method includes positioning the cryostat in such a disposition that said stream is supplied generally downwardly.
The present invention further consists in a nozzle for a cryogas cryostat comprising a central feed tube for supplying a stream of cryogas over a specimen, and an annular feed opening for supplying a dry annular flow of cryogas at ambient temperature surrounding said stream in the vicinity of the specimen, characterised in that the stream of cryogas is a stream of helium, that the dry annular flow of cryogas is also helium, and that said cryogas cryostat also comprises means for supplying said stream of helium at a temperature of between 4.2 and 77.4 degrees Kelvin to the central feed tube and means for supplying helium at ambient temperature to the annular feed opening.
Preferably in the vicinity of the specimen there is no solid barrier between said stream and said annular flow.
Preferably also the annular feed opening is surrounded by a shield extending beyond a mounting point for the specimen.
Preferably further the shield is beryllium having a high thermal conductivity and good thermal capacity to remain above freezing point in use.
Alternatively the shield is optically transparent and the nozzle has means to supply dry ambient temperature air along the outside of the shield in the vicinity of the specimen.
Preferably the annular feed opening is connected from a corresponding annular feed chamber.
Preferably further the annular feed chamber and annular feed opening connected to it comprise a radially inner wall which is of insulating material.
Preferably further the interior of the nozzle is a vacuum chamber.
The present invention also consists in a cryostat comprising a cryostat nozzle according to any one of the preceding statements of invention.
Preferably further the cryostat is provided with a source of cryogas to be connected to the central feed tube and operable at less than 77.4 degrees Kelvin and is provided with a dry source of the same cryogas at ambient temperature to be connected to the annular feed chamber.
Other preferred features of the invention will be apparent from the following description and from the subsidiary claims of the specification.